There are numerous measuring while drilling tools and some of them use a position control valve to encode the measurement into output signals by presure pulse modulating the mud flow. One such apparatus for pressure pulse modulating the mud flow has a valve member immersed in the mud flow and movably mounted to be displaced into a constriction in the mud flow to temporarily change back pressure in the drill string mud flow. Movement of this valve member is accomplished by a valve actuator that must displace the valve member in the mud flow stream in opposition to pressure forces created by the mud flowing downward within the drill string around the instrument and flowing upward within the borehole annulus. In some mechanical configurations of this valve assembly and its associated actuator the presure conditions of the mud can possibly prevent the valve from being properly displaced by the valve actuator and thus prevent the desired transmission of data from the measurement while drilling downhole instrument to the earth's surface through the mud flow path. Depending upon the mechanical configuration used the problems associated with pressure conditions affecting this valve assembly can result in inaccurate or erractic data transmission. Under certain pressure conditions the valve member can be pressure locked thus immovable and when this occurs no data transmission occurs.